Okay, what is it with my weird obsession with Phinabella sex scenes? It's starting to creep me out, because this is the third time that I've gotten random inspiration for one (the one based off real life doesn't count cause it actually happened). Whatever, they're still fun to write. Phineas and Isabella are 18, a couple, living together, not married, and this is not related to my other stories. I'd say this is the most explicit yet. Enjoy!

"Don't stress about it Phineas, you've got nothing to worry about."

Phineas and Isabella lay on their bed together side by side, both dressed only in their underpants. What had started out as some harmless kissing had eventually turned into this, but no one complaining. Well, not at first, but now there was something that Isabella had asked that made Phineas nervous.

"But what if I hurt you? I've never done this before."

"You won't hurt me. I trust you."

Phineas gazed into her eyes and saw there that she really did trust him, enough to let him do this by instinct alone. Isabella smiled, and that gave him enough courage to try for her sake.

"Alright, I'll try. But if I start hurting you then tell me right away."

"Of course."

Phineas took a deep breath, and then rolled over so that he was now lying on top of Isabella. She laughed and waited for him to continue. He took another deep breathe and slowly slid his hand down, across her stomach, hesitating for a second at her waist. All it took was one more look in her eyes to force his hand to move, sliding her underwear off. She blushed, but said nothing, still smiling wide. His hand lingered there, and then he placed it down, slowly moving his fingers around. Isabella closed her eyes and sighed.

"Wow Phineas," she said, her smile widening. Phineas took that as a sign to continue. He was completely unsure of how to go about this without hurting her, so he trusted his instincts and slid his hand lower. Isabella's breath turned heavier as he stopped his hand, and then, taking a third deep breath, slowly slid his index finger in.

Isabella let out a sharp exhale, making Phineas nervous.

"You okay?" he asked, not moving his hand away. She half opened her eyes, a smile still on her face.

"Perfect," she breathed. "Keep going."

Phineas only hesitated for a second before sliding his finger in deeper, which made Isabella close her eyes again and start breathing even heavier. Once again trusting his instincts, he started moving his finger in and out at a steady pace. Isabella gasped and he saw her squeeze her eyes shut tightly. Phineas instantly stopped the motion.

"Isabella?" he said nervously.

"You're a natural Phineas," she breathed. "Don't stop now."

A small smile found it's way to Phineas' face as he continued. Apparently, when it looked like she was in pain, it meant she really liked it. Phineas started to move his finger faster, and found himself enjoying it almost as much as her. He couldn't really describe the feeling it brought to him, but he definitely liked it. Isabella was nearly panting now, her face almost looking like it was in pain, except for the small smile, which made Phineas keep going. He slowed down for a second to slide his middle finger in as well, waiting to see her reaction. Isabella moaned loudly, but the smile never left her face, so Phineas started going faster again, and Isabella let out a small squeak.

"Oh God Phineas," she moaned, which made him slow down.

"Did that hurt?" he asked, concerned. Isabella opened her eyes slightly, smiling wider.

"A little, but it feels good," she answered.

"It does?"

"Yes. Please don't stop now."

Phineas couldn't entirely understand how something could hurt and feel good at the same time, but he wasn't a girl, so he did as she asked, resuming his original speed, and then going even faster. Small sounds kept escaping Isabella's lips, but she never asked him to stop. Phineas found himself not thinking about it anymore, just allowing himself to do whatever felt right. And Isabella liked everything he did. She tilted her head back, her breathe becoming labored, but she loved it. The breathless feeling only added to the experience, making it all the better. After a few minutes, she gasped, feeling herself climax. It hurt, but it felt so amazing that she loved it. Phineas seemed to pick up on what was going on, slowing down slightly to make her more comfortable. When the feeling passed, she sighed.

"Thank you Phineas."

Phineas slowly came to a stop and pulled his fingers out, which were now rather wet, but he knew that was fine. Isabella continued to breath heavily, her eyes still closed.

"How was it?" he asked, curious as to what she thought of what he had done. She opened her eyes and smiled wide.

"Amazing."

"You want me to do it again sometime?" he asked, blushing. Isabella pulled him down towards her for a kiss.

"Yes, yes I do."

I warned you! And now I'd love to hear what you thought. :)